Look in the PDF that is linked on the ArduinoInfo.info WIKI. Pin 1 is marked.

Pin numbers go Counter-Clockwise from pin 1. As seen from the TOP.

This is because people are used to going clockwise with pin numbers. On vacuum tubes. From the BOTTOM of the socket. Inside the chassis where you are soldering the filament transformer wires etc etc. In 1949. Everybody knows that

What I don't under stand is why you changed from transistor to Logic Output Optoisolators

If you think it works better you can still hook it up backward and 2 I showed you clearly how this worked with a transistor.

Actually, I'll probably try it both ways just to learn. I like the simplicity of the opt-isolator though, as it appears I don't have to worry but any other ground connections, etc. It just seems simpler to me. Certainly appreciate your help as well. I already have 2 NPN transistors (1 for backup, lol), so they will probably get first shot.

So, I decided to breadboard out my setup for testing, by seeing if I could toggle the remote board via momentary button connected to the Arduino and breadboard, and have the state of that button change the I/O on the Arduino to High for the pin I connected the NPN base to, whose collector and emitter are connected to the remote board HIGH and LOW in the location where the remote's momentary button is connected (see below):

And, I'm proud to say, it works like a charm! Yes! Ignore my solder job, I'm still trying to perfect it.

Gonna try optos next, just for fun, but I'm ready to package this up and integrate it with the rest of my setup, and into my Android app now.